Energy crisis and environmental pollution are two key issues that constrain the sustained development of current human society. Since solar energy is inexhaustible, clean and pollution-free and water is one of the most abundant substances on earth, if the solar energy can be utilized to split water efficiently and safely to release oxygen and obtain electrons and protons, to thereby generate electrical energy or hydrogen energy, the energy crisis and environmental pollution issues that human faces can thus be solved fundamentally. Furthermore, because water is a thermodynamically very stable chemical substance, it is necessary to provide a suitable water splitting catalyst to achieve efficient and safe water splitting. Recently, an international research team has used ions of Ru, Ir and other metal with some complex ligands to synthesize artificial catalysts having water-splitting function. However, all the reported catalysts do not have high catalytic efficiency in water splitting and need the presence of a strong oxidant (such as Ce(NH4)2(NO3)6) to split water. In addition, these known artificial catalysts, which use either noble metals or complex ligands, could lead to high preparation cost and easily cause environmental pollution and thus cannot be popularized and applied. Therefore, there is still an unsolved scientific problem about how to obtain an efficient, inexpensive and environmentally friendly water splitting catalyst.
The photosystem II of photosynthetic organisms is the only biological system in nature that be able to utilize inexpensive metal ions (Mn, Ca) efficiently and safely to achieve water splitting, obtain electrons and protons, and release oxygen at the same time. The key reason why the photosystem II is able to split water efficiently and safely is that it has a unique Mn4Ca cluster as the biological water splitting catalyst. Recent high-resolution study of the three-dimensional crystal structure of the photosystem II has found that the biological water splitting catalyst has the core of an asymmetric [Mn4CaOn] (n value dependent on the redox state of catalyst, which can be 4 or 5) heteronuclear metal cluster, which is formed by a O2− bridging between a Mn3CaO4 cubic alkane and a Mn ion. The biological water splitting catalyst at its periphery are provided with the ligands of six carboxyl groups, one imidazole and four water molecules. During the water splitting process, the biocatalyst undergoes five different states (S0, S1, S2, S3, S4). Among them, the valence states of the four manganese ions in the dark steady state (S1 state) are +3, +3, +4 and +4, respectively. The water splitting biocatalytic center of the photosystem II provided an ideal blueprint for the development of an inexpensive, efficient, and environmentally friendly artificial water splitting catalyst. Currently, how to chemically synthesize and prepare those similar to the biological water splitting catalytic center is an important scientific frontier and also a very challenging scientific problem. In this regard, no successful case has been reported yet.
The present invention hereby provides a novel process by a two-step synthesis using inexpensive metal ions (Mn2+, Ca2+ ions), a simple organic carboxylic acid and MnO4− as starting materials, to give an asymmetric [Mn4CaOn] core structure formed by an O2− bridging between a Mn3CaO4 cubic alkane and a Mn ion. The peripheral ligands of the [Mn4CaO4] consists of eight carboxyl anions and three exchangeable neutral ligands. The valence states of the four manganese ions are +3, +3, +4 and +4, respectively. These compounds have structures very similar to the biological water splitting catalytic center. Furthermore, we have found that these compounds also have physical and chemical properties similar to those of the biological water splitting catalytic center. Such compounds can catalyze the splitting of water to release oxygen in the presence of oxidant and can transfer the electrons released by the splitting of water to the surface of the electrode to form current. This type of compounds and their derivatives obtained by structural modification can be used as artificial catalysts for water splitting.